DNA damage–induced cell-cycle arrest of hematopoietic cells is overridden by activation of the PI-3 kinase/Akt signaling pathway

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Abstract
Exposure of hematopoietic cells to DNA-damaging agents induces cell-cycle arrest at G1 and G2/M checkpoints. Previously, it was shown that DNA damage–induced growth arrest of hematopoietic cells can be overridden by treatment with cytokine growth factors, such as erythropoietin (EPO) or interleukin-3 (IL-3). Here, the cytokine-activated signaling pathways required to override G1 and G2/M checkpoints induced by γ-irradiation (γ-IR) are characterized. Using factor-dependent myeloid cells stably expressing EPO receptor (EPO-R) mutants, it is shown that removal of a minimal domain required for PI-3K signaling abrogated the ability of EPO to override γ-IR–induced cell-cycle arrest. Similarly, the ability of cytokines to override γ-IR–induced arrest was abolished by an inhibitor of PI-3K (LY294002) or by overexpression of dominant-negative Akt. Moreover, the ability of EPO to override these checkpoints in cells expressing defective EPO-R mutants could be restored by overexpression of a constitutively active Akt. Thus, activation of a PI-3K/Akt signaling pathway is required for cytokine-dependent suppression of DNA-damage induced checkpoints. Together, these findings suggest a novel role for PI-3K/Akt pathways in the modulation of growth arrest responses to DNA damage in hematopoietic cells.